1. Technical Field
The present invention relates to a process for producing an inner race for a constant velocity joint, having improved cold workability, machinability, rolling fatigue life and flexural strength, as a boron-containing automobile component.
2. Background Art
SCM 420 steel (see JIS G 4105 (1979)) or a steel corresponding to SCM 420 steel has hitherto been carburized, quenched, and tempered before use for an inner race for an automobile constant velocity joint. In recent years, however, a reduction in weight of components is necessary for realizing an improvement in fuel consumption from the viewpoint of global environmental problems. On the other hand, working properties such as cold forgeability and machinability should not be sacrificed from the viewpoint of reducing the production cost. To this end, the development of an inner race for a constant velocity joint having improved rolling fatigue life and flexural strength without sacrificing cold forgeability and machinability is necessary.
The present inventors have now found that, in order to improve the rolling fatigue life, the abnormal-carburizing layer should be reduced in a rolling face not subjected to polishing and, in order to improve the flexural strength, the brittle fracture region should be reduced. Based on such finding, the present inventors have found optimal additive elements and amounts of added elements while taking into consideration cold workability and machinability, and have further found a suitable range of an effective case depth at the time of carburization/quenching, which has led to the completion of the present invention.
Accordingly, it is an object of the present invention to provide an inner race using SCM 420 (JIS) or a steel corresponding to SCM 420 for a constant velocity joint for the automobiles or the like, having improved rolling fatigue life and flexural strength properties without sacrificing cold forgeability and machinability.
According to the present invention, there is provided a process for producing an inner race for a constant velocity joint having improved workability and strength properties, said process comprising the steps of:
providing a steel comprising by weight carbon (C): 0.10 to 0.25%, silicon (Si): 0.03 to 0.15%, manganese (Mn): 0.20 to 0.60%, sulfur (S): 0.003 to 0.030%, chromium (Cr): 1.00 to 1.50%, titanium (Ti): 0.05 to 0.20%, boron (B): 0.0005 to 0.0050%, and nitrogen (N): not more than 0.01% with the balance consisting of iron (Fe) and inevitable impurities;
hot rolling or hot forging the steel, and optionally subjecting the steel to softening heat treatment, to prepare a rolled/forged product having a Rockwell hardness of not more than 75 HRB;
cold forging and machining the rolled/forged product to impart a shape of the inner race for a constant velocity joint; and
carburizing, quenching, and tempering the machined rolled/forged product so that the treated steel satisfies an effective case depth of 0.4 to 0.9 mm, a thickness of abnormal-carburizing layer of not more than 15 xcexcm, and an austenite grain size number as specified in JIS G 0551 of not less than 7, said effective case depth being defined as the distance from the surface of the steel to the position of the steel in its depth direction where the Vickers hardness is HV 550.
In the present invention, an improvement in rolling fatigue life in a rolling face not subjected to polishing has been made by reducing the contents of silicon and manganese having high affinity for oxygen in the chemical composition of the steel and reducing an abnormal-carburizing layer which has notch effect and is further soft. Further, since silicon and manganese are ferrite strengthening elements, reducing the contents of these elements can improve cold workability and machinability.
Furthermore, according to the present invention, boron is added to improve grain boundary strength, and, in addition, titanium is added to form titanium carbide or titanium carbonitride which suppress the growth of grains at the time of carburization after cold working to bring the austenite grain size number specified in JIS to not less than 7. This has reduced brittle fracture.
Furthermore, according to the present invention, the limitation of the effective case depth of the carburized layer defined as the distance from the surface of the steel to the position of the steel in its depth direction, where the Vickers hardness is HV 550, has reduced flexural load stress concentrated on the carburized layer and brittle cracking in an early stage at the time of flexural stress loading. This could realize increased strength.
Specifically, the present invention is directed to a process for producing an inner race for a constant velocity joint shown in FIG. 1 for automobiles or the like, having improved rolling fatigue life and flexural strength properties without sacrificing cold workability and machinability, using a steel product having the above chemical composition.